The ability to accurately measure in vivo tendon forces would have a broad impact on assistive technologies as well as our scientific understanding of the human neuromuscular control system. Myoelectric prosthetics and functional electrical stimulation devices could utilize closed-loop control strategies to restore motor function in disabled populations. From a basic sciences perspective, researchers could accurately study soft tissue properties, neuromuscular function, and motor performance directly, rather than having to rely on inaccurate, numerical approximations of these systems. The long-term objective of our proposed research is to develop a commercial device that accurately measures in vivo tendon forces with minimal invasiveness. Fiber optic technology is well suited for this application because of its microscopic size and high sensitivity. Furthermore, fiber optics sensors can easily be made biocompatible, require no external energy sources, and are immune to electromagnetic interference effects such as those found within most imaging systems like MRI. We propose that optical fiber sensors based on micro-bend fibers and fiber Bragg gratings will prove superior to the bare plastic fibers used in previous investigations. We will assess the performance of each of these fiber configurations during bench-top mechanical testing on human cadavers tendons. Using an Instron(r) material testing device, known tensile loads will be applied to the tendon, while corresponding voltages emitted from the sensor will be recorded. The performance of each fiber configuration will be based on numerous mechanical and electrical outcome measures. Using the optimal configuration determined during Phase I, we plan to test this sensor in both animal and human preparations as part of a Phase II SBIR proposal. The target user for the Stage I sensor developed during our proposed research will be academic and commercial laboratories where the sensor can be inserted, the experiments performed, and the sensor removed. Future development of a Stage II sensor will collapse the necessary electronics onto a micro-size chip that can be implanted for chronic monitoring of in vivo tendon forces, and can potentially interface directly with various assistive devices.